Definify.com
Webster 1913 Edition
Directrix
1.
A directress.
[R.]
Jer. Taylor.
2.
(Geom.)
(a)
A line along which a point in another line moves, or which in any way governs the motion of the point and determines the position of the curve generated by it; the line along which the generatrix moves in generating a surface.
(b)
A straight line so situated with respect to a conic section that the distance of any point of the curve from it has a constant ratio to the distance of the same point from the focus.
Webster 1828 Edition
Directrix
DIRECTRIX
,Noun.